Application Note pH in Brine Treatment & Dechlorination Chemical Industry: Chlor-Alkali The chlor-alkali process involves the electrolysis of NaCl fouled thus reducing their lifespan and driving up electrical brine. The reaction is shown below: costs. Chemical addition at the clarifi er is used to precip- itate out these impurities. The need for pH measurement

2NaCl + 2H2O ► Cl2 + H2 + 2NaOH here will be dictated by the quality of the NaCl available.

The resulting chlorine gas, hydrogen, and caustic from this After the initial treatment any further precipitated impurities process form the building blocks for many well known end as well as excess brine crystals are removed through a products. series of fi lters. The primary fi lter is typically a sand fi lter. After fi ltration the brine receives fi nal purifi cation by ion • Plastics (Nylons, PVC, Polycarbonates) exchange. The pH going into the ion exchange is typically • Pesticides measured to avoid damaging the resins. • Paint additives • Disinfectants (Sodium Hypochlorite) Depending on the design of the chlor-alkali facility it is • Surfactants (Soaps and shampoos) common to perform a fi nal acidifi cation of the brine before entering the electrolytic cell. Hydrochloric acid is used for Since the chlor-alkali plant provides feedstock to the this purpose and the reading is maintained between 4 to 6 fi nished chemical plant it will often be on-site or close-by. pH. Lowering the pH helps to remove any excess Cl- ions Accurate, reliable pH measurement is critical to ensuring which could damage the cell. effi cient operation of the chlor-alkali facility. This paper will explore this application and make recommendations on Dechlorination proper pH sensor selection. After electrolysis has occurred the spent brine still has value to the facility. It is put into a recirculation loop to go Brine Treatment back through the brine treatment process. Hydrochloric The creation of purifi ed brine to feed into the electrolytic Acid is again used to remove any free chlorine from the cell is a multi-step process. NaCl and water are mixed to brine. Control is typically at 2 to 4 pH prior to entering the create a saturated brine solution. The NaCl will contain dechlorination vacuum tower. A fi nal addition of NaOH 2+ 2+ 2- 2+ 2+ trace amounts of Mg , Ca , SO4 , Sr , and Ba . If these may be required downstream from dechlorination thus contaminants were to make it to the electrolytic cell then necessitating a fi nal pH measurement before return to the the membrane and related anode / cathode could be brine saturation tank.

Chlor-Alkali Brine Generation Circuit

HCl Acid Addition Cl2 H2 4pH 60°C (Cathode) (Anode) Clarifier Membrane (Chemical Addition) Brine Electrolytic Cell Saturation 9.5 to 11pH 60°C

Ion Primary Secondary Exchange Filtration Filtration

- + SO4 Ca Mg+

Dechlorination Brine NaOH Heater Depleted Brine Recirculation

NaOH 2 to 4 pH Addition 80°C HCl Acid Addition Figure 1 Application Note pH in Brine Treatment & Dechlorination

Measurement Challenges Most measurements found in brine purifi cation stages are pH measurement of the brine is diffi cult for multiple rea- accomplished using 3/4” or 1” sample line off the main sons. During the purifi cation phases, impurities can plug process. The Barben 546 sensor with 3/4” NPT threads the porous reference junction of the pH sensor. Precipitat- work well for these installations. If a 1” sample line is used ed heavy metals such a barium and strontium attack the the Barben 551 Quick Change sensor is also an option. Ag/AgCl reference element within the sensor thus causing These sensors are illustrated below. high offsets in the reading. The presence of chlorine, a strong oxidizing agent can also damage the reference. 546 & 551 Sample Line Sensors 3/4" NPT 3/4" PIPE TEE Throughout the process the elevated temperature and high P/N: B4951-0057 ionic strength of the brine can etch the glass electrode. All 546 SENSOR these factors combine to increase calibration intervals and KYNAR BODY shorten the pH sensor lifespan. SPECIFY INSERTION 1" WITH PLASTIC TEE SPECIFY FLUSH JUNCTION Barben Analyzer Technology’s Performance Series pH NUTLOCK FITTING P/N: B4953-0002 (HAND TIGHT) P/N: B4953-0015(HEX NUT) sensors are specifi cally designed to deal with the harsh 1” PIPE TEE ® 1" MNPT THREAD P/N: B4951-0046 chlor-alkali process. The patented Axial Ion Path solid 3.5 INCH INSERTION state reference cell acts as a fi lter to prevent precipitat- LENGTH 551 SENSOR ed brine impurities from clogging the porous reference. KYNAR BODY Calibration / cleaning intervals can be extended as fouling of the Barben probe is much more diffi cult than traditional FITTING NUT Figure 3 multi-junction pH designs. The same Axial Ion Path® refer- The dechlorination stages after the electrolytic cell may ence design also effectively blocks strong chemicals and use similar sample line measurements. In some heavy metals from penetrating into the sensor. This en- chlor-alkali facilities, retractable “hot tap” sensors are sures that the Ag/AgCl reference element is not poisoned selected. These sensors are mounted directly into the and the sensor output remains stable. process. This type of installation provides quick response Figure 2 when compared with sample line pH measurements. The Barben 547 hot tap cartridge style sensor with titanium Multiple Axial Ion Paths sheath and Kynar isolation valve provide the corrosion re- Plug free communication Seal individual fi ltering chambers sistance needed to survive in these applications. Barben can provide the sensor and all related hardware to make Annual Filtering Junction this installation successful. Maintains measurement signal Wood slows process ingress Highly resistant to strong chemicals 547 Hot Tap Retractable Sensor 1-1/4” OR 1-1/2“ FULL PORT KYNAR BALL VALVE Tefl on Junction Interface Initial protection against process PIPE NIPPLE KYNAR COMPRESSION FITTING

+ Large surface area + H H+ H H+ 547 SENSOR H+ H+ TITANIUM SHEATH H+

H+

Figure 4 ION PATH® pH Sensor Selection For all chlor-alkali applications “R” or “CR” high tempera- ture glass electrodes should be specifi ed. Both electrodes are well suited for both low & high pH applications found in the brine loop. The CR glass provides a secondary Barben Analytical reserves the right to make technical changes or modify the contents of this coating resistant layer thus is well suited for measurement document without prior notice. We reserve all rights in this document and in the subject matter at the clarifi er as well as caustic addition downstream and illustrations contained within. Hastelloy® is a registered trademark of Haynes Intl Inc. from the dechlorination tower. Kynar (PVDF) should be Kalrez® and Viton are registered trademarks of DuPont Dow Elastomers L.L.C. specifi ed as the sensor body material due to it’s chemical Kynar® is a registered trademark of Elf Atochem North America Inc. ® compatibility and integrity at elevated temperatures. Tefl on is a registered trademark of E.I. DuPont de Nemours Company Inc.

© 2013, by AMETEK, Inc. All rights reserved • Chloralkali_AN_RevA• July 2013 USA • BELGIUM • CHINA • SINGAPORE Toll Free +1(800)993-9309 • Phone +1(775)883-2500 • Fax +1(775)297-4740 [email protected] • www.BarbenAnalytical.com ISO 9001:2008 Cert. No. 43271